This Invention was made with Government support under Contract No. DE-FC26-01NT41020 awarded by the Department of Energy. The Government has certain rights in this invention.
The present invention relates to gas turbine engine combustors and, more particularly, to can-annular combustors with pre-mixers.
Industrial gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power for powering an electrical generator, for example. Electrical power generating gas turbine engines are typically operated for extended periods of time and exhaust emissions from the combustion gases are a concern and are subject to mandated limits. Thus, the combustor is designed for low exhaust emissions operation and, in particular, for low NOx operation. A typical low NOx combustor includes a plurality of combustor cans circumferentially adjoining each other around the circumference of the engine. Each combustor can has a plurality of pre-mixers joined to the upstream end. Lean burning pre-mixed low NOx combustors have been designed to produce low exhaust emissions but are susceptible to combustion instabilities in the combustion chamber.
Diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000.degree. F. and combines with oxygen to produce unacceptably high levels of NOx emissions. One method commonly used to reduce peak temperatures and, thereby, reduce NOx emissions, is to inject water or steam into the combustor. However, water/steam injection is a relatively expensive technique and can cause the undesirable side effect of quenching carbon monoxide (CO) burnout reactions. Additionally, water/steam injection methods are limited in their ability to reach the extremely low levels of pollutants required in many localities. Lean pre-mixed combustion is a much more attractive method of lowering peak flame temperatures and, correspondingly, NOx emission levels. In lean pre-mixed combustion, fuel and air are pre-mixed in a pre-mixing section and the fuel-air mixture is injected into a combustion chamber where it is burned. Due to the lean stoichiometry resulting from the pre-mixing, lower flame temperatures and NOx emission levels are achieved. Several types of low NOx emission combustors are currently employing lean pre-mixed combustion for gas turbines, including can-annular and annular type combustors.
Can-annular combustors typically consist of a cylindrical can-type liner inserted into a transition piece with multiple fuel-air pre-mixers positioned at the head end of the liner. Annular combustors are also used in many gas turbine applications and include multiple pre-mixers positioned in rings directly upstream of the turbine nozzles in an annular fashion. An annular burner has an annular cross-section combustion chamber bounded radially by inner and outer liners while a can burner has a circular cross-section combustion chamber bounded radially by a single liner.
Industrial gas turbine engines typically include a combustor designed for low exhaust emissions operation and, in particular, for low NOx operation. Low NOx combustors are typically in the form of a plurality of combustor cans circumferentially adjoining each other around the circumference of the engine, with each combustor can having a plurality of pre-mixers joined to the upstream ends thereof. Each pre-mixer typically includes a cylindrical duct in which is coaxially disposed a tubular centerbody extending from the duct inlet to the duct outlet where it joins a larger dome defining the upstream end of the combustor can and combustion chamber therein.
A swirler having a plurality of circumferentially spaced apart vanes is disposed at the duct inlet for swirling compressed air received from the engine compressor. Disposed downstream of the swirler are suitable fuel injectors typically in the form of a row of circumferentially spaced-apart fuel spokes, each having a plurality of radially spaced apart fuel injection orifices which conventionally receive fuel, such as gaseous methane, through the centerbody for discharge into the pre-mixer duct upstream of the combustor dome.
The fuel injectors are disposed axially upstream from the combustion chamber so that the fuel and air has sufficient time to mix and pre-vaporize. In this way, the pre-mixed and pre-vaporized fuel and air mixture support cleaner combustion thereof in the combustion chamber for reducing exhaust emissions. The combustion chamber is typically imperforate to maximize the amount of air reaching the pre-mixer and, therefore, producing lower quantities of NOx emissions and thus is able to meet mandated exhaust emission limits.
Lean pre-mixed low NOx combustors are more susceptible to combustion instability in the combustion chamber which causes the fuel and air mixture to vary, thus, lowering the effectiveness of the combustor to reduce emissions. Lean burning low NOx emission combustors with pre-mixers are subject to combustion instability that imposes serious limitations upon the operability of pre-mixed combustion systems. There exists a need in the art to provide combustion stability for a combustor which uses pre-mixing.
A gas turbine engine combustor can assembly includes a combustor can downstream of a pre-mixer having a pre-mixer upstream end, a pre-mixer downstream end, and a pre-mixer flowpath therebetween. A plurality of circumferentially spaced apart swirling vanes are disposed across the pre-mixer flowpath between the upstream and downstream ends. A primary fuel injector is used for injecting fuel into the pre-mixer flowpath. The combustor can has a combustion chamber surrounded by an annular combustor liner disposed in supply flow communication with the pre-mixer. An annular trapped dual vortex cavity is located at an upstream end of the combustor liner and is defined between an annular aft wall, an annular forward wall, and a circular radially outer wall formed therebetween. A cavity opening at a radially inner end of the cavity is spaced apart from the radially outer wall and extends between the aft wall and the forward wall. Air injection first holes are disposed through the forward wall and air injection second holes are disposed through the aft wall. The air injection first and second holes are spaced radially apart and fuel injection holes are disposed through at least one of the forward and aft walls.
An exemplary embodiment of the combustor can assembly includes angled film cooling apertures disposed through the aft wall angled radially outwardly in the downstream direction, film cooling apertures disposed through the forward wall angled radially inwardly, and film cooling apertures disposed through the outer wall angled axially forwardly. Alternatively, the film cooling apertures through the aft wall are angled radially inwardly in the downstream direction, the film cooling apertures through the forward wall are angled radially outwardly in the downstream direction, and the film cooling apertures through the outer wall are angled axially aftwardly. Each of the fuel injection holes is surrounded by a plurality of the air injection second holes and the air injection first holes are singularly arranged in a circumferential row. The primary fuel injector includes fuel cavities within the swirling vanes and fuel injection holes extending through trailing edges of the swirling vanes from the fuel cavities to the pre-mixer flowpath.
One alternative combustor can assembly has a reverse flow combustor flowpath including, in downstream serial flow relationship, an aft to forward portion between an outer flow sleeve and the annular combustor liner, a 180 degree bend forward of the vortex cavity, and the pre-mixer flowpath at a downstream end of the combustor flowpath. The swirling vanes are disposed across the pre-mixer flowpath defined between an outer flow sleeve and an inner flow sleeve. Another alternative combustor can assembly has a second stage pre-mixing convoluted mixer located between the pre-mixer and the vortex cavity. The convoluted mixer includes circumferentially alternating lobes extending radially inwardly into the pre-mixer flowpath.